MAX1530_09 Datasheet, PDF (29/33 Pages) Maxim Integrated Products – Multiple-Output Power-Supply Controllers for LCD Monitors

Multiple-Output Power-Supply Controllers for
LCD Monitors
Flying Capacitors
Increasing the flying capacitor value lowers the effec-
tive source impedance and increases the output cur-
rent capability. Increasing the capacitance indefinitely
has a negligible effect on output current capability
because the internal switch resistance and the diode
impedance place a lower limit on the source imped-
ance. A 0.1µF ceramic capacitor works well in most
low-current applications. The voltage rating for a given
flying capacitor (CX) must exceed the following:
V_CX > N x V_IN
where N is the stage number in which the flying capaci-
tor appears, and V_IN is the input voltage of the step-
down regulator.
Charge-Pump Output Capacitors
Increasing the output capacitance or decreasing the
ESR reduces the charge pump output ripple voltage
and the peak-to-peak transient voltage. With ceramic
capacitors, the output voltage ripple is dominated by
the capacitance value. Use the following equation to
approximate the required capacitor value:
C_OUT
≥
I_LOAD
2f_OSCV_RIPPLE
where V_RIPPLE is the peak-to-peak value of the output
ripple.
Charge-Pump Rectifier Diodes
Use low-cost silicon switching diodes with a current rat-
ing equal to or greater than 2 times the average
charge-pump input current. If it helps avoid an extra
stage, some or all of the diodes can be replaced with
Schottky diodes with an equivalent current rating.
Linear Regulator Controllers
Output Voltage Selection
Adjust the positive linear regulator (LR1 to LR4) output
voltages by connecting a resistive voltage-divider from
the output to AGND with the center tap connected to
FBL_ (Figure 1). Select the lower resistor of the divider
in the 10kΩ to 30kΩ range. Calculate the upper resistor
with the following equation:
[^{( )} ] ^RUPPER ^{= R}LOWER ^{× V}OUT_ ^{/ V}FBL_ ^−1
where V_FBL_ is 1.238V (typ).
Adjust the negative linear regulator (LR5) output volt-
age by connecting a resistive voltage-divider from
V_GOFF to VL with the center tap connected to FBL5
(Figure 1). Select R29 in the 10kΩ to 30kΩ range.
Calculate R28 with the following equation:
[ ] ^{( ) R28 = R29 × V}FBL5 ^{− V}GOFF^{) /(V}L ^{− V}FBL5
where V_FBL5 = 125mV and V_L = 5.0V.
Pass Transistor Selection
The pass transistor must meet specifications for DC
current gain (h_FE), collector-emitter saturation voltage,
and power dissipation. The transistor’s current gain lim-
its the guaranteed maximum output current to:
I_LOAD(MAX)
=
⎛
⎝⎜^IDRV
−
V_BE
R_BE
⎞
⎠⎟
×
h_FE
where I_DRV is the minimum guaranteed base drive cur-
rent, V_BE is the base-emitter voltage of the pass transis-
tor, and R_BE is the pullup resistor connected between
the transistor’s base and emitter. Furthermore, the tran-
sistor’s current gain increases the linear regulator’s DC
loop gain (see the Stability Requirements section),
which may destabilize the output. Therefore, transistors
with current gain over 300 at the maximum output cur-
rent can be difficult to stabilize and are not recom-
mended unless the high gain is needed to meet the
load current requirements.
The transistor’s saturation voltage at the maximum out-
put current determines the minimum input-to-output
voltage differential that the linear regulator supports.
Also, the package’s power dissipation limits the usable
maximum input-to-output voltage differential. The maxi-
mum power dissipation capability of the transistor’s
package and mounting must exceed the actual power
dissipation in the device. The power dissipation equals
the maximum load current (I_LOAD(MAX)) times the maxi-
mum input-to-output voltage differential:
P = I_LOAD(MAX) × (V_LRIN(MAX) − V_LROUT)
where V_LRIN(MAX) is the maximum input voltage of the
linear regulator, and V_LROUT is the output voltage of the
linear regulator.
Output Voltage Ripple
Ideally, the output voltage of a linear regulator should
not contain any ripple. In the MAX1530/MAX1531, the
step-down regulator’s switching noise can couple to
the linear regulators, creating output voltage ripple.
Following the PC board layout guidelines in the PC
Board Layout and Grounding section can significantly
reduce noise coupling. If there is still an unacceptable
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